"Where military response is concerned, we have a duty and a responsibility to ensure that where our armed forces are involved, the risks to them are quantifed and minimised. We cannot shelve or abandon that requirement.

"That means supporting American actions only in the knowledge that Britain will be involved in all planning and risk assessment," Mr Kennedy said.

There was a case for American military action, but with feelings understandably running high, the US risked going too far. "That's where a candid friend comes in. Standing shoulder to shoulder, but always there, for the occasional cautionary tap on the shoulder."

Earlier, Menzies Campbell, the party's foreign affairs spokesman, said military action in response to the terrorist attacks on New York and Washington must be based on "justice not revenge". He stressed that any allied action should be "precise and proportionate to the need" and consistent with international law.

But there was only muted applause when he said a military response based on those principles was justified and the UK should make forces available for that purpose.

Mr Campbell made clear that the public should expect there to be casualties among British troops. "I want you to understand that in committing forces in this way, we are accepting the risk of casualties.

"Easy to say in the autumn calm of a seaside town; more difficult to explain to grieving relations in the dawn at RAF Lyneham," said Mr Campbell.

He cautioned critics of military action not to resort to "crude anti-Americanism". No grievance, either real or perceived, justified such slaughter of human life in the United States. "The cause of this attack was not American foreign policy but an amoral disregard for human life."

The debate revealed deep anxieties among delegates about the nature of the American military response.

Donnachadh McCarthy, from Camberwell and Peckham, said Liberal Democrats, as "the political conscience of this country", must insist that all non-violent approaches were exhausted before military intervention.

Jenny Tonge, the party's international development spokesman, said the people of Afghanistan were facing poverty and starvation. She said: "We must bomb this area - but we must bomb it with food and aid. It's cheaper than military action and may win over a lot more hearts and minds."

Paul Keetch, the party's defence spokesman, said it was essential that the West did not fall into the terrorists' trap by making a knee-jerk overreaction to the atrocities. He added: "The blanket bombing of Afghanistan will achieve nothing save to make those intent on committing acts of terror all the more desperate and all the more aggressive."

To applause, the emergency motion was passed unanimously. It expressed sympathy for the victims of the attacks, which were described as a "crime against humanity" and affirmed support for the British Government's response.

But the motion also insisted that any action taken be "precise and proportionate", consistent with international law and "avoid as far as possible civilian casualties".